I believe the issue with the schrader valve/no schrader valve is the compression is lost due to 2 stroke exhaust cycle and pressure is exhausted out of test hose vs with schrader valve it is contained
Dear Steve, thanks a lot for your videos, I follow you closely from Denmark. I have a compressiontester without a valve. Do you Think i can buy such a valve somewhere?
I believe the one with a schrader valve holds the compression in the hose so it doesn't have to rebuild that pressure back up, whereas the one without loses the compression out the exhaust each time the piston opens up the exhaust port.
There is a check (Schrader) valve in both - the difference is the location. With tiny cylinders, the volume of the hose averages the cylinder pressure, instead of picking the peak. In large cylinders it matters much less. If the hose volume is same or even larger than the cylinder volume, the peak reading drops down to half or less of the plain cylinder peak. Same hose volume would probably be just 5% or less of the big cylinder volume, and the averaging effect is basically eliminated.
Well you had me worried there. I immediately ran out to the garage to check my tester. Fortunately it had the Schrader valve. I had tested a chainsaw at 45 PSI and after seeing what you did I wanted to make sure I hadn't condemned it improperly. Another fine lesson from Steve's Small Engine Saloon!
Every one of us without exception will be checking all of our compression testers today. Still working on coffee this sunday morning it can wait till later.
I have had lower end comp testers from Sears that did not work on small engines because they did not seal up enough on lower rpm engines. They would work fine on a car though. Before condemning, try the tester on a lawnmower just to make sure.
For your viewers, I bought the Mityvac compression tester exactly like yours and it is awesome. Good to mention too that it comes with 5 spare shrader valve replacements and replacement O rings. Can not remember what I paid for it in Canadian dollars, but it was worth every penny. Cheers Steve
Yep your right steve. The valve in the automotve tester is near the gauge. Basically what is happing is that when you pull the cord the second time, the air from the hose is pulled back into the engine. The small engine can only displace a small amount of air and is constantly draining and filling the hose as you have pointed out
I've always used the Schrader valve hose for all my testing's. Even automotive use. The only time I remove the valve is when I'm doing a compression leak down test. Great explanation Stevo.
Soooo many people argue constantly with me that they are all the same ! That compression is compression…. Wrong ! I believe you will find if you use the schrader valve compression tester with a shorter hose (ECHO makes a good one) you will have to pull it less times to get to your top psi. Was told by my boss years ago that the schrader valve itself is not the same you would find in a car tire, they apparently have a much lower spring rating as to not affect the pressure.. Thank you Steve for this !!!!!! Keep on !
There are "snifter valves" used in plumbing to prevent vacumm inside pipes and to allow some air to enter water pressure tanks that do not have a membrane or bladder to separate the air from the water. Those valves can have (at least) a 3 PSI spring core and 0 PSI core without spring at all, the internal pressure is the only thing that keeps the valve closed and the vacuum opens them. The core used to keep air pressure on bladder tanks has a 60 psi spring. "X PSI spring" means that the valve with allow air to enter when the air pressure reaches X PSI without anything pressing on the tip, the valve core opens on its own due to that air pressure pushing the seat down, assuming the pressure on the internal side of the valve is zero, if it's higher you must add the internal pressure to the spring pressure and if the pressure on the internal side of the valve is negative you have to substract that value from the spring pressure. Of course using a normal inflator that pushes down the tip the valve opens it no matter what the internal and external pressures are.
I had no idea..... This will come in handy one day and save me a lot of trouble (and time...and money). Thank you, Steve- This is simply the BEST small engine channel out there!
Well what know about the subject matter with my 35 years of experience of rebuilding all types of engines is as follows, the compression tester with no shrader valve should always read 0 when the engine is not cranking. There is nothing in the hose to hold any pressure at the gauge. The one with shrader valve will hold the pressure in the hose. Both tests have their own place. You have to remember that when an engine run it only gets one compression cycle per stroke cycle weather is 2 or four cycle. The one with the shrader valve reads max compression.
I have an old SNAP-ON compression tester kit that I purchased from a retired mechanic many years ago and have only used it sparingly on various engines of my own over the years. It has the Schrader valve in it....I didn't realize they made a tester without this valve? Learn something every day. Thanks Steve.....good explanation!
The automotive tester has a similar one-way valve, but it is all the way up at the gauge end of the tester, so all the dead space in the hose never lets the air expressed from the tiny cylinder in the saw get up to the actual pressure at the spark plug hole.
Your theory on why is spot on. The hose is adding to the chamber volume at TDC, thus having more influence on the pressure with the smaller cylinder, effectively lowering the compression ratio (CR). The bottom Schrader (check) valve keeps accumulated pressure from escaping the hose and so not lowering CR after the pressure trapped in the hose maxes out. A shorter or smaller diameter hose made of material that doesn't easily expand would help, as it reduces the loss in CR, but the check valve at the spark plug hole is the best way to get an accurate reading no matter what size the cylinder is.
All compression testers have a check valve in the line somewhere, but the red-box version makes the mistake of putting it in the pipe just below the gauge instead of out at the end of the hose. Why? because that way, they only need one valve for the whole kit instead of one valve per line. Lots of kits have this right, but the photos on Amazon don't usually tell you where the valve is located. And yes, Steve, you got it right about the reason why the red-box kit doesn't work for a small-displacement engine. That added volume in the hose is enough to reduce the compression pressure dramatically. By the way, when the Schrader valve in the end of a compression tester goes south (and they will, eventually), it's not safe to assume that the ones for tires will work as replacements. What you need is a Schrader with a really low spring tension so that the compression will pop it open really easily. Some tool suppliers sell them, and you can usually pick them up from the guy on the tool truck if you work in a shop. The ones sold on Amazon for that purpose are not always right, though; hit up someplace that really does tools instead.
Just bought a Mityvac from Amazon . Works perfect . I wonder how many Testers have been sold because of your video Steve . You should get a commission. Thanks again Steve , You are a STAR.
I literally just had this problem couldn't figure out why the cheaper gauge wouldn't work. Now it make sense. Thank you. I bought an OTC set that one works great
That’s absolutely right. The pressure in the hose needs to build to match that inside the cylinder. I discovered this when I was testing a lawnmower. The Schrader valve was loose and each pull you could see the pressure go up, then slowly go down. Once I tightened the valve, the pressure would build and hold. You must have the Schrader valve for small engines.
Yeah, I got the tester in the red case. Leaf blower read 25 psi but still ran, couldn't understand it till 8 found out about the Schrader valve. What I don't understand is why do you have to buy a whole new tester? Why can't you just buy a replacement tube only with the schrader valve?
I think you can, I have 3 gauges and 2 Hose sets, Both have the schrader valve and both hose interchange with each other, so you might be able to buy a new hose with the schrader valve in it.
I think you are exactly right! Without the Schrader valve, you are effectively lowering the compression ratio quite drastically. Which lowers compression dramatically. Think about increasing your combustion chamber to twice as large as it is. Your compression would drop a bunch! By adding a bunch of hose volume, it's effectively making your combustion chamber much larger, hence lower compression. Great job, great video, and excellent information. I've never given this a thought before, I guess, because luckily my gage has a Schrader valve. Thanks!
I have been master ase certified for 40 years. I suspect that the tester without the schraeder in the hose actually does have a check valve, probably a schraeder, in the guage body. However, I would caution, if using engine oil to temporarily reseal rings for a second test on a cylinder with low compression, make sure to inject the oil through the spark plug hole then crank the engine to belch out the excess oil before installing the tester. The purged oil can be caught on a shop rag. This second test was known as a dry/wet compression test and comes from the days when the labor to perform a tuneup included a complete compression test to help avoid spending money buying tuneup parts trying to fix an engine mechanical problem. I sometimes use oil to boost compression on a tired engine that cranks but wont start as a diagnostic aid and i haven't killed a tester since the first two after starting the excess oil purge method.
Yeah, man you a totally right that no valve version compresses extra air and that’s why it shows the moon phase instead of what we dare to check. Great observation! Thanks Steve!
Legend as always. Knew there's a difference and have a tester i only use for small engines but it's good to see your video and people talking about it in the comments. Happy Xmas, merry new year and peas n luv to you and yours. Xx
I always wonder about that. Same issue, around 40 lbs, but it ran great. At first, I thought it was the decompression value, but this is the first video I've seen that explains the correct compression tester with a schrader valve. Great video. Thanks
Steve, I learned this lesson the hard way a year ago. I purchased a compression tester without the schrader valve to test on my small engines. Results: low compression. Then I borrowed a compression tester that happened to have the schrader valve and all my small engine equipment tested fine. Of course, your video confirmed what I had suspected.
now ain’t that about something as I was doing a compression test this morning on a Briggs engine and now finding out about the shader valve on a compression tester. very informative thank you good job brother 💪
45psi????? I about spit my beer out on that one! That old Husky sounded like it should have a kicker on it......haha. Good stuff man. Thank you, I didnt even realize they made a tester without the valve, other than the tapered rubber based units. Every time I stop in here I learn something new. Merry Christmas and Happy New Year to all!
Steve, it would have been helpful if you would have connected the schrader valve hose to the 45 PSI gauge and eliminate a bad gauge. There was an air fitting on it so it would have been easy. I agree with your assessment as to why it was so different.
Yup, everything you said is right on! You're usually using the starter to turn over larger engines as opposed to hand cranking the smaller ones and that too helps with the non-Schrader Valve types being more accurate for larger engines. The Schrader valve types really are the only type to use for either if you want accuracy without second guessing every reading. Just like a quality tire pressure gauge vs. one from the dollar store for $1.99. The cheap one could be off by any amount and hardly any of them actually give the correct reading.
We'll, I could have used this information about three years ago. I had 40 psi on my Toro Tecumseh engine and I just pushed it to the road and bought a new one.
Great info Steve! I've known this for a long time, but some of the cheapie ones are terribly inaccurate even with the Schrader valve. No regrets about buying that Mityvac a while back. You are absolutely correct about the added volume of the hose. I just did the math: a 24" long hose, 1/4" in diameter will add over 19cc of volume!!!
I am 76. I have NEVER seen a compression tester with no schrader in the plug end. I STILL HAVE my old Proto I bought sometime in the '70's, along with others from garage sales, etc
Some have a ball style check valve on the gauge quick connet side and pressurizing the hose on every stroke is too much for a small engine. Maybe the manufacturers are afraid of the schrader valve core getting loose and falling inside a car engine.
Thank yo so much Steve, this video explains a problem I had with 2 Victa 2 stroke engines . I decided to have both engines Rebored and Honed and fitted New Pistons and rings. I was disappointed to find I was only getting 45 psi after all that time trouble and expense. This video explains why I was only getting 45 psi on the tester. Need to buy a new Tester. Thanks again Steve, thought I was going nuts.
@@Nozinbonsai Yes the Victa does have a decompression valve however ,You just need to remove it and screw in a spare spark plug to block the decompressor.
I bought an almost new Craftsman compression tester from a person on Marketplace for 30.00. It has all the hoses and different attachments for different spark plug sizes. Best investment I've made.
It's very simple, the hose effectively lowers the compression ratio, its like having more cc's in the head spacing, or dishing the piston. By having the valve right at the start of the hose the pressure in the hose itself won't bleed back, so you can eventually fill up the hose to the actual compression of the cylinder.
Both systems hold the pressure being fed from the cylinder in the tube Steve. It's just that the system with the valve stops it bleeding straight back into cylinder where it will leak out (2 stroke?) on every stroke. Whereas it won't leak back past the Schroeder when it's NOT under a near constant supply of pressure pulses in a pull start engine. The fix would be to purchase an air-hold adapter. I only went and ordered a non Schroeder kit just a few hours ago.
I've got the automotive type and never had an issue but when the weather gets better I'm gonna add a schrader valve to mine and see if it improves my readings.
The volume of the hose is added to the swept volume. It's also added to the combustion chamber volume, so it has to effect the potential compression reading. There should always be a valve at the point where the plug is to know what pressure will be in the combustion chamber. Interesting they are the same with larger volume cylinders and not compensated for the added volume of the tube.
I think you’re correct on volume being the issue with the non-Schrader valve tester. Timely video as I was going to purchase one soon. Thanks for the tip and have a Happy New Year Steve.
I think I might have to disagree about where the pressure is being measured and why one is different. I think both measure pressure all the way through the hose and up to the display meter, but the one with the valve prevents the compressed air from entering back into the head and leaking out through the piston and rings. When the engine is running there isn't time for the compressed air and gasses to leak out past the rings, but while testing there is plenty of time for the compressed air to leak back through the rings. My opinion. Thanks Steve for another video, I really enjoy your channel.
Great info! The quench ratio is greatly chsnged eith small cc compairrd to your truck engine. Also remember when replacing the shrader valve it takes a different one than a regular old tire valve stem.
I bought a compression tester from Amazon (which I have not opened or used yet) to replace two older ones without a Schrader valve, and the dial cover on one was faded and cracked. It has a pressure release button, but it's on the meter connection-end of the hose, so it may be off from small engines. I'll have to see if it's too late to return it. I rented one like it from AutoZone before the purchase and it showed the compression increased to 130psi after lapping the valves, where it was around 80psi before, but I would rather have a compression tester that I know it accurate for small engines. My Tractor Mower's Single OHV Briggs is burning oil now. Either the valve gaps need to be adjusted again or the valve guide that moved and wouldn't budge has done something bad. I need to check compression again, but that can wait until next Spring, so I have time to buy the Mityvac MV5530 in your link. $73.67 is a lot, but peace of mind is worth it, and your affiliation link will help you. Thank you, Steve!
That is a great video. I went to my tool box to look at mine. Mine has the schrader valve at the quick change coupler. I think mine is a Cal-van brand. I bought some brass adapters to check compression on troy bilt 4 cycle weed eaters. Small spark plugs Just has a hole that goes through it. Check the other compression tester hose and see for yourself if no schrader exists. Have a happy new year!
This issue with the cheap compression testers and low readings is common with small motorcycle engines (particularly 2 strokes where the piston is a fairly regular service item).
I'm glad someone finally touched on this. I've been trying to get a decent compression tester for about 2 years cant seem to get a good one for under 100 dollars Canadian.
nailed it, plus the Schrader valve does not have to keep "re"bulding the pressure to the guage with each stroke. *the Maddox deluxe compression tested at harbor freight is identical to the mityvac one at about $25 less and you can get it on sale for cheaper than that all the time
Yes you are correct. The compression ratio is determined by the combustion space above the piston at TDC. For that 65cc Husky the volume is likely to be near 6cc above the piston at TDC.. If you add 10cc of hose, then you have a volume of 16cc, which you are cramming the 65cc of the Husky into.
My automotive / diesel engine compression tester kit has a schrader valve. I bought it over 3 decades ago when I started as a mechanic. Come to think of it, I have never seen one without a schrader valve.
I've had a mighty vac vacuum tester since the 80 or 90's. Works great but it dont get used much anymore with newer vehicles. I need a new compression tester Thanks for this video. I work on small engines as well as cars and trucks but had no idea of the differences until now. Thanks again.
Removing the Schrader was done for 2 reasons. 1, to use for a running compression test and 2, for checking head gasket leak down with shop air. Running compression used to be an advanced diagnostic that we did after finding issues with vacuum gauges under load and at running temp. These tests can now be done with a Pico scope electrically. Most cheaper gauges were not accurate enough for these tests to show the fluctuations at running rpm.
So glad someone made this video! Also.. Don't buy a Harbor Freight one! Always reads low. I bought one a LONG time ago at Auto Zone or Advance Auto I think. Has the valve. Even had to change it, started leaking. Has to be a special one just for this purpose. Yes it's the hose "adding" to the displacement of the engine. I test motorcycles also, wonder at what size they would test the same.
And if you replace the Schrader don't use a tire valve because it has a heavy spring. It will read low. A tire chuck has a pin in the middle that opens the Schrader. The one specifically for the compression testers have a very light spring
Tried to reply with a link to the Schrader valve core pdf but it keeps getting deleted. The censors do not want us to know the details about schrader valves. Yes we need the snifter type for this application.
By adding the volume of the testers hose you have added combustion chamber volume. This effectively drops the compression ratio. Steve, you are right , the volume of that hose added to such a small combustion chamber will drop that reading. I have seen this happen to various degrees even on small displacement automobile engines too. Take the internal volume of that hose and add it directly to the combustion chamber volume, you can drastically drop the engines compression ratio.
Thanks brotha, to even have seen this I would have had to use somebody elses or bought another and then by chance seen the difference. I now see the extra shrader valve cores on the OTC amazon page.
Steve, there are two options to correct this on compression testers that do not have a low inlet pressure schrader valves. First is to locate a low inlet pressure schrader valve and thread it into the end of your compression tester end fittings. Locating schrader valves with 1-4 lbs of inlet pressure may be a little difficult and there is always the question of can the schrader valve be threaded in. Schrader Pacific makes such valves but you would have to call them for a list of their distributors. Second, my favorite, is to simply fit a check valve (for air of course) with a low inlet pressure as close to the changeable spark plug fitting end as possible. Just make sure the check valve is rated for 250psi or higher. McMaster-Carr or even Amazon has such valves. Both options can be repair options also to resurrect an old but favored compression tester.
It's the volume in the hose that's the difference. It will make a difference on a car engine too, but not significant. My rule of thumb on a car engine is that if all cylinders are the same then the compression is most likely not the problem unless the cam has jumped or something. Add to it that those gauges are often quite inaccurate (could be 10% off). But 10% off won't make a diagnostic difference.
Thank you Steve. Now I know why one of my small engines runs fine even though the compression tested at about 40 PSI. I used an automotive compression tester.
Also good to know to look for that 40PSI level when testing with the non-schrader type. No need to rush out and buy another tester right away but for next time we will know better.
Excellent point about the valve vs. no valve on the compression testing!! Really appreciate the pro tip! Thanks so much for sharing this one!!! Say HI to April also... When the weather gets warmer I will check out which tester I have. Stay warm up in Canada!!! ⛄❄
I wonder, the metal valve stems used on a truck tire has the 1/4 NPT on base end and the 1/8 NPT where the cap fits on could this be cobbled together ? The metal valve stem could be had from your local tire store.
Many thanks for this video Steve, and greetings of the season to April and yourself from the UK. yes I did buy the wrong one, and yes I did take the chainsaw and outboard apart because of reading around 40psi. Every day's a school day.
I just checked my tester, and had a big "oh no..." moment.🤦🏼♂️😖. No schrader valve. 💸 I hate throwing money away on tools! Steve, thank you for posting this great info 👍🏼, 🍻🇨🇦
I agree with an earlier reply in that it would have been if interest to have used the same gauge for both tests. While the valve is needed to obtain a useful reading on small engines, you can't discount the impact that a garbage gauge from China will have on the test. "Bargain" Chinese pressure gauges (ESPECIALLY like dial type ones used for tire pressure purposes) are NOTORIOUSLY INACCURATE!!! A CHEAP tire gauge OR compression tester gauge is subject to provide you with GROSSLY inaccurate readings! I speak from experience...I was swayed by price and bought a compression tester kit from Scamazon. After using it and getting what I thought to be bogus readings, I removed the gauge and installed it alongside a good quality air gauge on a manifold bar. After attaching to my air compressor at a regulated 100 lbs. of pressure, the cheap gauge showed 71 psi while the quality gauge showed the full 100 psi. Not really a surprise, the error % amount varied depending on the regulated air pressure. At 35 psi (typical passenger tire pressure) the cheap gauge registered 22 psi. Lower pressures were even far less accurate! You buy a Chinese air gauge (or mechanical torque wrench, it's s total crap shoot at best...the quality control at a majority of their cheap tool/cheap device manufacturers is totally non-existant!!! MANY THANKS for taking the time to produce your channel...always very informative and presented in a very relatable manner!!!
the small engines need the Schrader because the length of line acts like a large reservoir in the combustion chamber volume which lowers the compression. I noticed the same!
✅ SMALL ENGINE COMPRESSION TESTER ➜ amzn.to/4gQb6Cw
✅ AUTOMOTIVE COMPRESSION TESTER ➜ amzn.to/4hcOYCL
👉 Drop Starting VIDEO ➜ ua-cam.com/video/PsF2SEWEa18/v-deo.html
I believe the issue with the schrader valve/no schrader valve is the compression is lost due to 2 stroke exhaust cycle and pressure is exhausted out of test hose vs with schrader valve it is contained
De
Dear Steve, thanks a lot for your videos, I follow you closely from Denmark. I have a compressiontester without a valve. Do you Think i can buy such a valve somewhere?
I believe the one with a schrader valve holds the compression in the hose so it doesn't have to rebuild that pressure back up, whereas the one without loses the compression out the exhaust each time the piston opens up the exhaust port.
I think that's it.
Bingo, exactly
There is a check (Schrader) valve in both - the difference is the location. With tiny cylinders, the volume of the hose averages the cylinder pressure, instead of picking the peak. In large cylinders it matters much less. If the hose volume is same or even larger than the cylinder volume, the peak reading drops down to half or less of the plain cylinder peak. Same hose volume would probably be just 5% or less of the big cylinder volume, and the averaging effect is basically eliminated.
I concur
Ya i was going to say everyone i have seen have a check valve somewhere
Well you had me worried there. I immediately ran out to the garage to check my tester. Fortunately it had the Schrader valve. I had tested a chainsaw at 45 PSI and after seeing what you did I wanted to make sure I hadn't condemned it improperly. Another fine lesson from Steve's Small Engine Saloon!
Every one of us without exception will be checking all of our compression testers today. Still working on coffee this sunday morning it can wait till later.
@@IslandwaterjetCoffee should ALWAYS be of the HIGHEST priority! 😂
Make sure the Schrader valve is tight. It could lose pressure and give a false reading. You want to see the gauge pressure build and hold.
I have had lower end comp testers from Sears that did not work on small engines because they did not seal up enough on lower rpm engines. They would work fine on a car though. Before condemning, try the tester on a lawnmower just to make sure.
For your viewers, I bought the Mityvac compression tester exactly like yours and it is awesome.
Good to mention too that it comes with 5 spare shrader valve replacements and replacement O rings.
Can not remember what I paid for it in Canadian dollars, but it was worth every penny.
Cheers Steve
Yep your right steve. The valve in the automotve tester is near the gauge. Basically what is happing is that when you pull the cord the second time, the air from the hose is pulled back into the engine. The small engine can only displace a small amount of air and is constantly draining and filling the hose as you have pointed out
I've always used the Schrader valve hose for all my testing's. Even automotive use. The only time I remove the valve is when I'm doing a compression leak down test. Great explanation Stevo.
Soooo many people argue constantly with me that they are all the same ! That compression is compression…. Wrong !
I believe you will find if you use the schrader valve compression tester with a shorter hose (ECHO makes a good one) you will have to pull it less times to get to your top psi. Was told by my boss years ago that the schrader valve itself is not the same you would find in a car tire, they apparently have a much lower spring rating as to not affect the pressure..
Thank you Steve for this !!!!!! Keep on !
There are "snifter valves" used in plumbing to prevent vacumm inside pipes and to allow some air to enter water pressure tanks that do not have a membrane or bladder to separate the air from the water. Those valves can have (at least) a 3 PSI spring core and 0 PSI core without spring at all, the internal pressure is the only thing that keeps the valve closed and the vacuum opens them.
The core used to keep air pressure on bladder tanks has a 60 psi spring.
"X PSI spring" means that the valve with allow air to enter when the air pressure reaches X PSI without anything pressing on the tip, the valve core opens on its own due to that air pressure pushing the seat down, assuming the pressure on the internal side of the valve is zero, if it's higher you must add the internal pressure to the spring pressure and if the pressure on the internal side of the valve is negative you have to substract that value from the spring pressure.
Of course using a normal inflator that pushes down the tip the valve opens it no matter what the internal and external pressures are.
I had no idea..... This will come in handy one day and save me a lot of trouble (and time...and money). Thank you, Steve- This is simply the BEST small engine channel out there!
Well what know about the subject matter with my 35 years of experience of rebuilding all types of engines is as follows, the compression tester with no shrader valve should always read 0 when the engine is not cranking. There is nothing in the hose to hold any pressure at the gauge. The one with shrader valve will hold the pressure in the hose. Both tests have their own place. You have to remember that when an engine run it only gets one compression cycle per stroke cycle weather is 2 or four cycle. The one with the shrader valve reads max compression.
Thanks Steve. Making the world a better place one video at a time.
Old dog here, learning new tricks! Never knew about the differences with small engines. Thanks Steve (good to see you back)! Happy New Year🇨🇦🇺🇸📆🎉
+1
I have an old SNAP-ON compression tester kit that I purchased from a retired mechanic many years ago and have only used it sparingly on various engines of my own over the years. It has the Schrader valve in it....I didn't realize they made a tester without this valve? Learn something every day. Thanks Steve.....good explanation!
The automotive tester has a similar one-way valve, but it is all the way up at the gauge end of the tester, so all the dead space in the hose never lets the air expressed from the tiny cylinder in the saw get up to the actual pressure at the spark plug hole.
Your theory on why is spot on. The hose is adding to the chamber volume at TDC, thus having more influence on the pressure with the smaller cylinder, effectively lowering the compression ratio (CR). The bottom Schrader (check) valve keeps accumulated pressure from escaping the hose and so not lowering CR after the pressure trapped in the hose maxes out. A shorter or smaller diameter hose made of material that doesn't easily expand would help, as it reduces the loss in CR, but the check valve at the spark plug hole is the best way to get an accurate reading no matter what size the cylinder is.
All compression testers have a check valve in the line somewhere, but the red-box version makes the mistake of putting it in the pipe just below the gauge instead of out at the end of the hose. Why? because that way, they only need one valve for the whole kit instead of one valve per line. Lots of kits have this right, but the photos on Amazon don't usually tell you where the valve is located. And yes, Steve, you got it right about the reason why the red-box kit doesn't work for a small-displacement engine. That added volume in the hose is enough to reduce the compression pressure dramatically.
By the way, when the Schrader valve in the end of a compression tester goes south (and they will, eventually), it's not safe to assume that the ones for tires will work as replacements. What you need is a Schrader with a really low spring tension so that the compression will pop it open really easily. Some tool suppliers sell them, and you can usually pick them up from the guy on the tool truck if you work in a shop. The ones sold on Amazon for that purpose are not always right, though; hit up someplace that really does tools instead.
Will car AC system schrader valves work?
Old mechanic completely agrees with you Steve Happy New Year.
Great seeing you back Steve , you’re always a must watch ! Thanks for sharing all your knowledge with us .
Just bought a Mityvac from Amazon . Works perfect . I wonder how many Testers have been sold because of your video Steve . You should get a commission. Thanks again Steve , You are a STAR.
I literally just had this problem couldn't figure out why the cheaper gauge wouldn't work. Now it make sense. Thank you. I bought an OTC set that one works great
That’s absolutely right. The pressure in the hose needs to build to match that inside the cylinder. I discovered this when I was testing a lawnmower. The Schrader valve was loose and each pull you could see the pressure go up, then slowly go down. Once I tightened the valve, the pressure would build and hold. You must have the Schrader valve for small engines.
Yeah, I got the tester in the red case. Leaf blower read 25 psi but still ran, couldn't understand it till 8 found out about the Schrader valve. What I don't understand is why do you have to buy a whole new tester? Why can't you just buy a replacement tube only with the schrader valve?
I think you can, I have 3 gauges and 2 Hose sets, Both have the schrader valve and both hose interchange with each other, so you might be able to buy a new hose with the schrader valve in it.
I think you are exactly right! Without the Schrader valve, you are effectively lowering the compression ratio quite drastically. Which lowers compression dramatically. Think about increasing your combustion chamber to twice as large as it is. Your compression would drop a bunch! By adding a bunch of hose volume, it's effectively making your combustion chamber much larger, hence lower compression.
Great job, great video, and excellent information. I've never given this a thought before, I guess, because luckily my gage has a Schrader valve.
Thanks!
I have been master ase certified for 40 years. I suspect that the tester without the schraeder in the hose actually does have a check valve, probably a schraeder, in the guage body. However, I would caution, if using engine oil to temporarily reseal rings for a second test on a cylinder with low compression, make sure to inject the oil through the spark plug hole then crank the engine to belch out the excess oil before installing the tester. The purged oil can be caught on a shop rag. This second test was known as a dry/wet compression test and comes from the days when the labor to perform a tuneup included a complete compression test to help avoid spending money buying tuneup parts trying to fix an engine mechanical problem. I sometimes use oil to boost compression on a tired engine that cranks but wont start as a diagnostic aid and i haven't killed a tester since the first two after starting the excess oil purge method.
Yeah, man you a totally right that no valve version compresses extra air and that’s why it shows the moon phase instead of what we dare to check. Great observation! Thanks Steve!
Legend as always.
Knew there's a difference and have a tester i only use for small engines but it's good to see your video and people talking about it in the comments.
Happy Xmas, merry new year and peas n luv to you and yours. Xx
I always wonder about that. Same issue, around 40 lbs, but it ran great. At first, I thought it was the decompression value, but this is the first video I've seen that explains the correct compression tester with a schrader valve. Great video. Thanks
Steve, I learned this lesson the hard way a year ago. I purchased a compression tester without the schrader valve to test on my small engines. Results: low compression. Then I borrowed a compression tester that happened to have the schrader valve and all my small engine equipment tested fine. Of course, your video confirmed what I had suspected.
It is amazing how much knowledge is in your head. I learn something new from you all the time. Thanks, Steve
You are right Steve! The mass of the air is spot on!
now ain’t that about something as I was doing a compression test this morning on a Briggs engine and now finding out about the shader valve on a compression tester. very informative thank you good job brother 💪
45psi????? I about spit my beer out on that one! That old Husky sounded like it should have a kicker on it......haha.
Good stuff man. Thank you, I didnt even realize they made a tester without the valve, other than the tapered rubber based units. Every time I stop in here I learn something new. Merry Christmas and Happy New Year to all!
Good God man, be careful. NEVER spit out beer!
@@richarddykowski1060 It was def a close one! haha
That "Amazon" one probably has a schrader valve, but it'll be by gage, so almost renders it ineffective.
Steve, it would have been helpful if you would have connected the schrader valve hose to the 45 PSI gauge and eliminate a bad gauge. There was an air fitting on it so it would have been easy. I agree with your assessment as to why it was so different.
Yup, everything you said is right on! You're usually using the starter to turn over larger engines as opposed to hand cranking the smaller ones and that too helps with the non-Schrader Valve types being more accurate for larger engines. The Schrader valve types really are the only type to use for either if you want accuracy without second guessing every reading. Just like a quality tire pressure gauge vs. one from the dollar store for $1.99. The cheap one could be off by any amount and hardly any of them actually give the correct reading.
Planning to buy one soon. Great timing. Thanks Steve
We'll, I could have used this information about three years ago. I had 40 psi on my Toro Tecumseh engine and I just pushed it to the road and bought a new one.
These small gas engines often have a compression release so even with the Schrader valve, they will still read low.
Great info Steve! I've known this for a long time, but some of the cheapie ones are terribly inaccurate even with the Schrader valve. No regrets about buying that Mityvac a while back. You are absolutely correct about the added volume of the hose. I just did the math: a 24" long hose, 1/4" in diameter will add over 19cc of volume!!!
Awesome! Thank You Stella
And we all laughed when our grade 5 math teacher told us we need to know how to calculate the volume of a cylinder.
Yes! Needed this video 3 years ago before I bought the wrong one.
I am 76. I have NEVER seen a compression tester with no schrader in the plug end. I STILL HAVE my old Proto I bought sometime in the '70's, along with others from garage sales, etc
Some have a ball style check valve on the gauge quick connet side and pressurizing the hose on every stroke is too much for a small engine.
Maybe the manufacturers are afraid of the schrader valve core getting loose and falling inside a car engine.
Glad you did this video on the 2 different types of commpression testers
Thank yo so much Steve, this video explains a problem I had with 2 Victa 2 stroke engines . I decided to have both engines Rebored and Honed and fitted New Pistons and rings. I was disappointed to find I was only getting 45 psi after all that time trouble and expense. This video explains why I was only getting 45 psi on the tester. Need to buy a new Tester. Thanks again Steve, thought I was going nuts.
Sometimes there's a decompressor which makes compression testing useless.
@@Nozinbonsai Yes the Victa does have a decompression valve however ,You just need to remove it and screw in a spare spark plug to block the decompressor.
Need all the real help we need to avoid some of my shop expenses; glad to learn from you Steve! Thnx much.
I bought an almost new Craftsman compression tester from a person on Marketplace for 30.00.
It has all the hoses and different attachments for different spark plug sizes.
Best investment I've made.
It's very simple, the hose effectively lowers the compression ratio, its like having more cc's in the head spacing, or dishing the piston. By having the valve right at the start of the hose the pressure in the hose itself won't bleed back, so you can eventually fill up the hose to the actual compression of the cylinder.
Another great video of exceptional information on the right tool for the job ..
Yes Steve much love and respect from the uk
Have a happy new year Steve and thanks for all the great advice mate from Britain.
Both systems hold the pressure being fed from the cylinder in the tube Steve. It's just that the system with the valve stops it bleeding straight back into cylinder where it will leak out (2 stroke?) on every stroke. Whereas it won't leak back past the Schroeder when it's NOT under a near constant supply of pressure pulses in a pull start engine. The fix would be to purchase an air-hold adapter.
I only went and ordered a non Schroeder kit just a few hours ago.
Thanks for the info! I had heard many automotive compression testers were not accurate on small engines, now I know what to look for.
I've got the automotive type and never had an issue but when the weather gets better I'm gonna add a schrader valve to mine and see if it improves my readings.
The volume of the hose is added to the swept volume.
It's also added to the combustion chamber volume, so it has to effect the potential compression reading.
There should always be a valve at the point where the plug is to know what pressure will be in the combustion chamber.
Interesting they are the same with larger volume cylinders and not compensated for the added volume of the tube.
Another Golden Nugget of Info 👍
I think you’re correct on volume being the issue with the non-Schrader valve tester. Timely video as I was going to purchase one soon. Thanks for the tip and have a Happy New Year Steve.
I have had the same problem on boat motor testing. I thought it was just bad guage. Thanks! 👍🏻😁
I think I might have to disagree about where the pressure is being measured and why one is different. I think both measure pressure all the way through the hose and up to the display meter, but the one with the valve prevents the compressed air from entering back into the head and leaking out through the piston and rings. When the engine is running there isn't time for the compressed air and gasses to leak out past the rings, but while testing there is plenty of time for the compressed air to leak back through the rings. My opinion. Thanks Steve for another video, I really enjoy your channel.
Great info! The quench ratio is greatly chsnged eith small cc compairrd to your truck engine. Also remember when replacing the shrader valve it takes a different one than a regular old tire valve stem.
I got that Mity Vac kit a while back. It is a little pricey but the variety of adaptors is great and very useful
I bought a compression tester from Amazon (which I have not opened or used yet) to replace two older ones without a Schrader valve, and the dial cover on one was faded and cracked. It has a pressure release button, but it's on the meter connection-end of the hose, so it may be off from small engines. I'll have to see if it's too late to return it. I rented one like it from AutoZone before the purchase and it showed the compression increased to 130psi after lapping the valves, where it was around 80psi before, but I would rather have a compression tester that I know it accurate for small engines. My Tractor Mower's Single OHV Briggs is burning oil now. Either the valve gaps need to be adjusted again or the valve guide that moved and wouldn't budge has done something bad. I need to check compression again, but that can wait until next Spring, so I have time to buy the Mityvac MV5530 in your link. $73.67 is a lot, but peace of mind is worth it, and your affiliation link will help you. Thank you, Steve!
There may be lots of different tools for the job, but they're not all the right tool for the job. Thanks Steve.
That is a great video. I went to my tool box to look at mine. Mine has the schrader valve at the quick change coupler. I think mine is a Cal-van brand. I bought some brass adapters to check compression on troy bilt 4 cycle weed eaters. Small spark plugs Just has a hole that goes through it. Check the other compression tester hose and see for yourself if no schrader exists. Have a happy new year!
This issue with the cheap compression testers and low readings is common with small motorcycle engines (particularly 2 strokes where the piston is a fairly regular service item).
I have the Mityvac. Haven't used it much but it seems to work good. Leon the Homelite guy has recommended it for a long time.
Thank you for your service Steve
Thanks Steve Happy New year
Spot on, Steve, as usual!!!!👍👍👍
I'm glad someone finally touched on this. I've been trying to get a decent compression tester for about 2 years cant seem to get a good one for under 100 dollars Canadian.
Yes I had both style testers, always thought the one was way off for some reason ! thanks alot !! no one has ever mentioned this before !
nailed it, plus the Schrader valve does not have to keep "re"bulding the pressure to the guage with each stroke.
*the Maddox deluxe compression tested at harbor freight is identical to the mityvac one at about $25 less and you can get it on sale for cheaper than that all the time
I've never though about it before but I think you are absolutely right!
I just ordered one on your recommendation!! I've had various luck using the red box one. Sometimes it gives me a good compression and sometimes not.
Yes you are correct. The compression ratio is determined by the combustion space above the piston at TDC. For that 65cc Husky the volume is likely to be near 6cc above the piston at TDC.. If you add 10cc of hose, then you have a volume of 16cc, which you are cramming the 65cc of the Husky into.
Don't see the point of that......
Exactly
My automotive / diesel engine compression tester kit has a schrader valve. I bought it over 3 decades ago when I started as a mechanic. Come to think of it, I have never seen one without a schrader valve.
Wow I didn't even know that was two pressure testers thank you for the video and happy New Years
I've had a mighty vac vacuum tester since the 80 or 90's. Works great but it dont get used much anymore with newer vehicles. I need a new compression tester Thanks for this video. I work on small engines as well as cars and trucks but had no idea of the differences until now. Thanks again.
Yep found out the hard way years ago on small outboard testing.
I’m very glad you have covered this problem because I’m tired of having to show them 😊😊😊😊
It has to do with volume. 100 percent. Good stuff, thank you
Removing the Schrader was done for 2 reasons. 1, to use for a running compression test and 2, for checking head gasket leak down with shop air. Running compression used to be an advanced diagnostic that we did after finding issues with vacuum gauges under load and at running temp. These tests can now be done with a Pico scope electrically. Most cheaper gauges were not accurate enough for these tests to show the fluctuations at running rpm.
So glad someone made this video! Also.. Don't buy a Harbor Freight one! Always reads low.
I bought one a LONG time ago at Auto Zone or Advance Auto I think. Has the valve. Even had to change it, started leaking. Has to be a special one just for this purpose.
Yes it's the hose "adding" to the displacement of the engine. I test motorcycles also, wonder at what size they would test the same.
Great video. I never considered this. Thanks.
And if you replace the Schrader don't use a tire valve because it has a heavy spring. It will read low. A tire chuck has a pin in the middle that opens the Schrader. The one specifically for the compression testers have a very light spring
Tried to reply with a link to the Schrader valve core pdf but it keeps getting deleted. The censors do not want us to know the details about schrader valves. Yes we need the snifter type for this application.
By adding the volume of the testers hose you have added combustion chamber volume. This effectively drops the compression ratio. Steve, you are right , the volume of that hose added to such a small combustion chamber will drop that reading. I have seen this happen to various degrees even on small displacement automobile engines too. Take the internal volume of that hose and add it directly to the combustion chamber volume, you can drastically drop the engines compression ratio.
Thanks brotha, to even have seen this I would have had to use somebody elses or bought another and then by chance seen the difference. I now see the extra shrader valve cores on the OTC amazon page.
Great info Steve!
Steve, there are two options to correct this on compression testers that do not have a low inlet pressure schrader valves. First is to locate a low inlet pressure schrader valve and thread it into the end of your compression tester end fittings. Locating schrader valves with 1-4 lbs of inlet pressure may be a little difficult and there is always the question of can the schrader valve be threaded in. Schrader Pacific makes such valves but you would have to call them for a list of their distributors. Second, my favorite, is to simply fit a check valve (for air of course) with a low inlet pressure as close to the changeable spark plug fitting end as possible. Just make sure the check valve is rated for 250psi or higher. McMaster-Carr or even Amazon has such valves. Both options can be repair options also to resurrect an old but favored compression tester.
Awesome thanks!
Awesome information Steve.
You're a great source of knowledge.
Thanks.
Happy New Year.🎉
It's the volume in the hose that's the difference. It will make a difference on a car engine too, but not significant.
My rule of thumb on a car engine is that if all cylinders are the same then the compression is most likely not the problem unless the cam has jumped or something.
Add to it that those gauges are often quite inaccurate (could be 10% off). But 10% off won't make a diagnostic difference.
Thank you Steve. Now I know why one of my small engines runs fine even though the compression tested at about 40 PSI. I used an automotive compression tester.
Also good to know to look for that 40PSI level when testing with the non-schrader type. No need to rush out and buy another tester right away but for next time we will know better.
You can also have an engine with a compression release...
@@patthesoundguy True but I don't own an engine with compression release.
Great video and great explanation.
Happy New Year to you and yours.
Excellent point about the valve vs. no valve on the compression testing!! Really appreciate the pro tip! Thanks so much for sharing this one!!! Say HI to April also...
When the weather gets warmer I will check out which tester I have. Stay warm up in Canada!!! ⛄❄
Excellent info. I'm hoping to find a schrader valve attachment to my tester. Hoping I don't have to buy a whole new kit.
I wonder, the metal valve stems used on a truck tire has the 1/4 NPT on base end and the 1/8 NPT where the cap fits on could this be cobbled together ? The metal valve stem could be had from your local tire store.
Many thanks for this video Steve,
and greetings of the season to April and yourself from the UK.
yes I did buy the wrong one,
and yes I did take the chainsaw and outboard apart because of reading around 40psi.
Every day's a school day.
I just learned something! Thank you, Steve 😅
I just checked my tester, and had a big "oh no..." moment.🤦🏼♂️😖. No schrader valve. 💸 I hate throwing money away on tools! Steve, thank you for posting this great info 👍🏼, 🍻🇨🇦
Best small engine tip I’ve heard for 2024
Sounds right to me Steve I take the Schrader valve out of my compression tester and tried it I got the same results that you did
Cool! Thank You...
I agree with an earlier reply in that it would have been if interest to have used the same gauge for both tests. While the valve is needed to obtain a useful reading on small engines, you can't discount the impact that a garbage gauge from China will have on the test.
"Bargain" Chinese pressure gauges (ESPECIALLY like dial type ones used for tire pressure purposes) are NOTORIOUSLY INACCURATE!!!
A CHEAP tire gauge OR compression tester gauge is subject to provide you with GROSSLY inaccurate readings!
I speak from experience...I was swayed by price and bought a compression tester kit from Scamazon. After using it and getting what I thought to be bogus readings, I removed the gauge and installed it alongside a good quality air gauge on a manifold bar. After attaching to my air compressor at a regulated 100 lbs. of pressure, the cheap gauge showed 71 psi while the quality gauge showed the full 100 psi.
Not really a surprise, the error % amount varied depending on the regulated air pressure. At 35 psi (typical passenger tire pressure) the cheap gauge registered 22 psi. Lower pressures were even far less accurate!
You buy a Chinese air gauge (or mechanical torque wrench, it's s total crap shoot at best...the quality control at a majority of their cheap tool/cheap device manufacturers is totally non-existant!!! MANY THANKS for taking the time to produce your channel...always very informative and presented in a very relatable manner!!!
Yep, I have been told the hose adds volume to the combustion chamber and gives you a wrong reading.
Great Christmas gift idea for next year. Thanks and Happy New Year, Steve!
I thought that (this year) 🙂
Very good info as always. Thanks.
the small engines need the Schrader because the length of line acts like a large reservoir in the combustion chamber volume which lowers the compression. I noticed the same!